High-speed plotters require a fast and highly controlled movement of a writing pen. Most commonly, the pen is supported on a carriage which is mounted on transverse guide rails for back and forth movement in a lateral direction while the plotter web, such as a paper sheet, is independently moved back and forth in a direction substantially perpendicular to the carriage movement. Motion along these two axes in combination with up/down pen control permits the creation of accurate line art drawings and lettering on the web.
Controlled up/down motion of the pen with respect to the web is critical for high-speed plotter operation. Present methods of pen control include either a solenoid or a force coil to effect pen up/down motion. However, the use of a solenoid makes it extremely difficult to control the pen motion to provide a soft landing of the pen tip against the web. The use of a force coil overcomes this difficulty, but mounting the force coil structure on a movable pen carriage adds substantial mass to the moving systems and requires electrical connection to the carriage. Furthermore, effective control of pen up/down motion using a force coil requires an expensive closed loop servo controller which requires pen position and velocity feedback.
Other methods of up/down pen control include systems where the actuation means are located remote to the carriage and mounted to one side on the plotter frame. The actuator is then connected to the carriage by way of a mechanism such as a push bar which extends across the entire width of the plotter and is parallel to the guide rails. Prior art mechanical systems such as these which are remote to the carriage, add complexity and extra mass to the moving systems, and therefore are slow. Furthermore, there is a problem with accuracy and repeatability since there is often misalignment between the push bar and the guide rails.
There is, therefore, a need in the plotter art for a method of fast and accurate up/down pen control which is economical and does not unduly increase the complexity of the plotter.